Method for preparing low-concentration polyaluminosilicate microgels

ABSTRACT

An improved method and apparatus for preparing low-concentration polyaluminosilicate microgels from a water soluble silicate and a strong acid in which the silicate and acid are mixed at a rate to produce a Reynolds number of at least 4000, the mixture is aged and then diluted to a silica concentration of not more than 1.0 wt. %. The method achieves reduced silica deposition during the preparation of the microgels.

RELATED U.S. APPLICATIONS

This application is a continuation-in-part of application Ser. No.08/345,890, filed Nov. 28, 1994, now U.S. Pat. No. 5,503,820, which wasa continuation-in-part of Ser. No. 08/166,679, filed Dec. 16, 1993,abandoned, which application was a continuation-in-part of U.S. Ser. No.08/093,157, filed Jul. 25, 1993, now U.S. Pat. No. 5,312,595, which wasa divisional of U.S. Ser. No. 02/887,793, filed May 26, 1992 now U.S.Pat. No. 5,279,807.

BACKGROUND OF THE INVENTION

The present invention relates to an improved method and apparatus forpreparing low-concentration polysilicate microgels, i.e., aqueoussolutions having an active silica concentration of generally less thanabout 1.0 wt. %, which are formed by the partial gelation of an alkalimetal silicate or a polysilicate, such as sodium polysilicate, having inits most common form one part Na₂ O to 3.3 parts SiO₂ by weight. Themicrogels, which are referred to as "active" silica in contrast tocommercial colloidal silica, comprise solutions of from 1 to 2 nmdiameter linked silica particles which have a surface area of at leastabout 1000 m² /g. The particles are linked together during preparation,i.e., during partial gelation, to form aggregates which are gaged intothree-dimensional networks and chains. The polysilicate microgels can befurther modified by the incorporation of aluminum oxide into theirstructure. Such alumina modified polysilicates are classified aspolyaluminosilicate microgels and are readily produced by a modificationof the basic method for polysilicate microgels. A critical aspect of theinvention is the ability to produce the microgels within a reasonabletime period, i.e., not longer than about 15 minutes until the microgelis ready for use, without the risk of solidification and with minimumformation of undesirable silica deposits within the processingequipment. In this connection, the incorporation of alumina into thepolysilicate microgel has been found beneficial in that it increases therate of microgel formation. Polysilicate microgels produced according tothe invention are particularly useful in combinations with water solublecationic polymers as a drainage and retention aid in papermaking. At lowpH values, below pH of 5, these products are more appropriately referredto as polysilicic acid microgels. As the pH value is raised, theseproducts can contain mixtures of polysilicic acid and polysilicatemicrogels; the ratio being pH-dependent. For sake of convenience, theseproducts hereinafter will be referred to as polysilicate microgels.

SUMMARY OF THE INVENTION

The present invention is an improved method and apparatus forcontinuously preparing a low-concentration polysilicate microgel whichcomprises: (a) simultaneously introducing a first stream comprising awater soluble silicate solution and a second stream comprising a strongacid having a pKa less than 6 into a mixing zone where the streamsconverge at an angle of not less than 30 degrees and at a ratesufficient to produce a Reynolds number of at least about 4000 and aresulting silicate/acid mixture having a silica concentration in therange of from about 1.0 to 6.0 wt. % and a pH in the range of from 2 to10.5; (b) aging the silicate/acid mixture for a period of timesufficient to achieve a desired level of partial gelation (i.e., formingthe microgel), usually for at least 10 seconds but not more than about15 minutes; and (c) diluting the aged mixture to a silica concentrationof not greater than about 2.0 wt. % whereby gelation is stabilized. Toproduce polyaluminosilicate microgels, a water soluble aluminum salt isadded first to the acid stream prior to mixing it with the silicatestream.

For best results, the silica concentration of the water soluble silicatestarting solution is in the range of from 2 to 10 wt. % silica, and theconcentration of the strong acid (e.g., sulfuric acid) is in the rangeof from 1 to 20 wt. % acid as the two streams are being introduced intothe mixing zone. The preferred conditions in the mixing zone are aReynolds number greater than 6000, a silica concentration in the rangeof 1.5 to 3.5 wt. % and a pH in the range of 7 to 10. The most preferredconditions are a Reynolds number greater than 6000, silica concentrationof 2 wt. % and a pH of 9. The preparation of alumina modified microgelis best conducted by adding a soluble aluminum salt to the acid streamin an amount ranging from about 0.1 wt. % up to the solubility limit ofthe aluminum salt. The most useful polyaluminosilicate microgels arethose prepared with an Al₂ O₃ /SiO₂ mole ratio ranging from 1:1500 to1:25 and, preferably, from 1:1250 to 1:50.

The apparatus according to the invention comprises: (a) a firstreservoir for containing a water soluble silicate solution; (b) a secondreservoir for containing a strong acid having a pKa of less than 6; (c)a mixing device having a first inlet which communicates with said firstreservoir, a second inlet arranged at an angle of at least 30 degreeswith respect to said first inlet which communicates with said secondreservoir, and an exit; (d) a first pumping means located between saidfirst reservoir and said mixing device for pumping a stream of silicatesolution from said first reservoir into said first inlet, and firstcontrol means for controlling the concentration of silica in saidsilicate solution while said solution is being pumped such that thesilica concentration in the exit solution from the mixing device is inthe range of 1 to 6 wt. %; (e) a second pumping means located betweensaid second reservoir and said mixing device for pumping a stream ofacid from said second reservoir into said second inlet at a raterelative to the rate of said first pumping means sufficient to produce aReynolds number within said mixing device of at least 4000 in the regionwhere the streams converge whereby said silicate and said acid arethoroughly mixed; (f) mixture control means located within said exit andresponsive to the flow rate of said acid into said mixing device forcontrolling the pH of the silicate/acid mixture in the range of from 2to 10.5; (g) a receiving tank; (h) an elongated transfer loop whichcommunicates with the exit of said mixing device and said receiving tankfor transferring said mixture therebetween; (i) a dilution means fordiluting the silicate/acid mixture in the receiving tank to a silicaconcentration of not more than 1.0 wt. %; (j) a fourth reservoir forcontaining a water soluble aluminum salt; (k) a fourth pumping devicefor introducing the aluminum salt into the acid stream; and (I) acontrol valve responsive to the aluminum salt flow and linked inparallel with the silicate control valve, and located between the fourthpumping device and the point of introduction of the aluminum salt intothe acid stream.

In an alternate embodiment, the apparatus of the invention includes aNaOH reservoir and means for periodically flushing the production systemwith warm NaOH which has been heated to a temperature of from 40° to 60°C. whereby deposits of silica can be solubilized and removed.

In a further embodiment of the invention, an agitating gas stream suchas a stream of air or nitrogen or other inert gas can be introduced intothe mixing device described by means of an additional inlet located ator near the mixing junction. Gas agitation provides an importantindustrial benefit in that it permits low silicate flow rates to beemployed while maintaining the required turbulence and Reynolds numberin the mixing zone.

In yet a further embodiment of this invention, mixing of the acid,aluminum salt and the water soluble silicate solution can beaccomplished in an annular mixing device. This device can be an internalpipe or tube which protrudes into and subsequently discharges inside ofa larger pipe or tube. The internal pipe discharge point is usually, butnot necessarily, concentrically located inside the external pipe. One ofthe two fluids to be mixed is fed into the internal pipe. The secondfluid is fed into the external pipe and flows around the outside of theinternal pipe. Mixing of the two fluids occurs where the first fluidexits the internal pipe and combines with the second fluid in the largerexternal pipe. Usually, the acid and the aluminum salt solution arepremixed prior to being fed into one of the pipes.

For the purpose of mixing the two liquids, the water soluble silicatesolution and the acid can be fed to either the internal or the externalpipes at rates sufficient such that when the two streams are combined, aReynolds number of greater than 4000 is produced in the mixing zone. Anagitating gas stream can also be optionally employed to aid in themixing of the two streams.

As a further embodiment to this invention, mixing of the acid and watersoluble silicate solution can be accomplished in a vessel equipped withmechanical means to create the necessary turbulence, such that mixing ofthe two streams is accomplished at a Reynolds number of greater than4000. The vessel can optionally be equipped with baffles. The acid andwater soluble silicate solution can be but do not have to be fed to thevessel simultaneously.

To produce polyaluminosilicate microgels, a concentrated solution of analuminum salt, preferably aluminum sulfate, is pumped from an additionalreservoir and mixed into the diluted acid stream at a point before thatat which the diluted acid and silicate streams are mixed and reacted. Bythe addition of the aluminum salt to the acid stream, the rate offormation of microgel is increased and a polyaluminosilicate microgel isformed having aluminum moieties incorporated throughout the microgelstructure.

The method and apparatus of the invention are capable of producingstable polysilicate and polyaluminosilicate microgels resulting inreduced silica deposition within a convenient time frame of not morethan about 15-16 minutes, but usually within 30 to 90 seconds, withoutthe risk of solidification and with minimum formation of undesirablesilica deposits within the processing equipment. Temperature ofoperation is usually within the range of 0°-50° C.

Silica deposition in production apparatus is undesirable because itcoats all internal surfaces of the apparatus and can impede thefunctioning of vital moving parts and instrumentation. For example,silica deposition can build to the point where valves can no longerfunction and can restrict fluid flow through pipes and tubing.Deposition of silica is also undesirable on the pH sensing electrode asit prevents monitoring the process pH, a critical quality controlparameter for silica microgel production.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the process which includes a NaOHreservoir and means for periodically flushing the production system.

FIG. 2 is a schematic diagram of a dual line polysilicate microgelproduction system which provides for uninterrupted microgel production.

FIG. 3 is a schematic diagram of the process of the invention for theproduction of polyaluminosilicate microgels which includes an aluminumsalt reservoir and means for introducing said salt into the dilute acidstream.

DETAILED DESCRIPTION OF THE INVENTION

Active silica is a specific form of microparticulate silica comprisingvery small 1-2 nm diameter particles which are linked together in chainsor networks to form three-dimensional structures known as "microgels".The surface area of the active silica microparticulates, i.e., themicrogels, is at least about 1000 m² /g. General methods for preparingpolysilicate microgels are described in U.S. Pat. No. 4,954,220, theteachings of which are incorporated herein by reference. Of the methodsdescribed therein, the acidification of a dilute aqueous solution of analkali metal silicate with an inorganic acid or organic acid, i.e., astrong acid having a pKa of less than 6, is the method to which thisinvention is particularly applicable. The present invention provides forthe reliable and continuous preparation of low-concentrationpolysilicate and polyaluminosilicate microgels at the site of intendedconsumption without formation of undesirable silica deposits within theprocessing equipment and at very reasonable aging times generally lessthan 15 minutes, and preferably between from 10 to 90 seconds.

The method of the invention is carried out by simultaneously introducinga stream of a water soluble silicate solution and a stream of strongacid having a pKa less than 6, along with an aluminum salt, into amixing zone or mixing junction such that the streams converge at anangle of generally not less than 30 degrees. with respect to each otherand at a rate which is sufficient to produce a Reynolds number in theregion where the two streams converge of at least 4000, and preferablyin the range of about 6000 and above. Reynolds number is a dimensionlessnumber used in engineering to describe liquid flow conditions within atube or pipe. Numbers below 2000 represent laminar flow (poor mixingenvironment) and numbers of 4000 and above represent turbulent flow(good mixing environment). As a general rule, the larger the Reynoldsnumber the better the mixing. Reynolds number, (Re) for flow in a pipeor tube, is determined from the equation ##EQU1## Where: Q=Flow in cubicfeet per second

d=Density in pounds per cubic foot

D=Pipe diameter in feet

u=Viscosity in pounds per foot second

Reynolds number for impeller-stirred vessels is determined from theequation

    Re=(D.sup.2 ×N×p)/u

Where:

D=Impeller diameter in cm

N=Rotational velocity in revolutions per second

p=Fluid density in grams per cm³

u=Viscosity in grams per (second)(centimeter)

The concentrations of the converging silicate solution and theacid/aluminum salt streams are controlled so that the resultingsilicate/acid mixture thus produced has a silica concentration in therange of 1 to 6 wt. % and a pH in the range of 2 to 10.5. Morepreferably the silica concentration is in the range of 1.5 to 3.5 wt. %and the pH is in the range of 7 to 10. The most preferred operatingconditions are with a Reynolds number larger than 6000, a silicaconcentration of 2 wt. % and a pH of 9.

Aging is generally accomplished in from 10 up to about 90 seconds bypassing the silicate/acid mixture through an elongated transfer loop inroute to a finished product receiving tank in which the mixture isimmediately diluted and thereafter maintained at an active silicaconcentration of not greater than 2.0 wt. % and, preferably, not greaterthan 1.0 wt. %. Partial gelation which produces the three-dimensionalaggregate networks and chains of high surface area active silicaparticles is achieved during aging. Dilution of the silicate/acidmixture to low concentration operates to halt the gelation process andstabilize the microgel for subsequent consumption.

The method of the invention and an apparatus for carrying it out willnow be discussed in greater detail in reference to the drawings in whichFIG. 1 is a schematic diagram of the process in its simplest form toprepare polysilicate microgels. The sizes, capacities and ratesdescribed herein can be varied over wide ranges depending primarily onthe quantities of polysilicate microgel required and the expected rateof consumption. The sizes and capacities described in reference to thedrawings relate to a system for producing, i.e., generating,polysilicate microgel on a generally continuous basis for consumption asa drainage and retention aid in a papermaking process in which theconsumption rate ranges from about 10 to 4000 lbs. microgel per hour.

There is shown in FIG. 1 a dilution water reservoir 10, an acidreservoir 12, and a silicate reservoir 14. The reservoirs, i.e., tanks,are conveniently made of polyethylene, with the water reservoir having acapacity of 500 gallons, the acid reservoir having a capacity of 100gallons, and the silicate reservoir having a capacity of 300 gallons.Other vessels shown in FIG. 1 are NaOH flush tank 16 and finishedproduct receiving tank 18. The NaOH flush tank is made of anon-corrosive material, such as, for example, 316 stainless steel; ithas a capacity of 20 gallons and is heated with an electrical resistancedrum heater wrapped around it (Cole-Palmer, 2000 watts, 115 volts). Thefinished product receiving tank has a capacity of 1000 gallons and ismade of polyethylene.

A critical element of the process is mixing junction 20 which defines amixing zone in which a stream of acid and a stream of water solublesilicate are introduced along individual paths which converge within themixing zone at an angle generally not less than 30 degrees. A mixing "T"or "Y" junction is suitable for practicing the invention and may readilybe constructed from an appropriately sized 316 stainless steel"Swagelok" compression coupling fitted with stainless steel tubing. A"T" junction is generally preferred.

The rates at which the two streams enter, i.e. are pumped into, themixing zone are selected to produce a Reynolds number therewithin of atleast 4000 and preferably up to 6000 or higher which results inpractically instantaneous and thorough mixing of the acid and silicatesuch that the resulting mixture has a silica concentration in the rangeof from 1.5 to 3.5 wt. % and a pH of from 7 to 10. Any convenientcommercial source of water soluble silicate can be employed, such as,for example, "PQ (N)" sodium silicate (41 Baume, SiO₂ :Na₂ O=3.22:1 byweight, 28.7 wt. % SiO₂) marketed by the PQ corporation. The commercialsilicate is maintained undiluted in reservoir 14, usually at aconcentration of 24 to 36 wt. % as supplied by the manufacturer, untilit is needed. It is supplied to the mixing junction 20 via suitabletubing 22 (316 SS, 1/4 inch OD) by means of a low flow rate gear ormicropump 24 (e.g., Micropump Corp., model 140, max. flow 1.7 gpm).Non-corrosive materials of construction, e.g., 316 stainless steel, arepreferred to avoid any risk of corrosion and subsequent contamination.The silicate supply line also includes flow control valve 26 (Whitey,316 SS, 1/4 inch needle), magnetic flow meter 28 (Fisher Porter, 316 SS,1/10 inch size) and check valve 86 (Whitey, 316 SS, 1/4 inch diameter)for controlling and monitoring the amount and direction of silicateflow. In operation, dilution water is introduced into the silicatesupply line 22 at a convenient location upstream of the silicate/acidmixing junction 20 so as to adjust the silica concentration to a valuein the range of from 2 to 10 wt. %. To insure complete mixing ofsilicate and water an in-line static mixer 32 (Cole-Palmer, 316 SS, 1/2inch tubing, 15 elements) is provided followed by a check valve 30(Whitey, 316 SS, 1/2 inch diameter). The dilution water is supplied vialine 34 (1/2 inch OD, 316 SS) by centrifugal pump 36 (Eastern Pump, 1HP, max. flow 54 gpm), and a rotameter 38 (Brooks, Brass Ball, 3.06 gpmmax.). Control valve 40 (Whitey, 316 SS, 1/2 inch NE needle) and checkvalve 42 (Whitey, 316 SS, 1/2 inch diameter) can be employed to thecontrol flow rate and direction.

Although a wide range of acidic materials, such as, for example, mineralacids, organic acids, acid salts and gases, ion-exchange resins and thesalts of strong acids with weak bases, have been described for use inpreparing active silica, the simplest and most convenient means ofacidification is with a strong acid having a pKa less than 6. Thepreferred acid is sulfuric acid. Commercial grades manufactured byDuPont and others are generally suitable. In operation, a stock solutionof acid is maintained at a concentration in the range of from 5 to 100wt. % in acid reservoir 12. The acid is pumped using a gear or similarmicropump 44 (e.g., Micropump model 040, 1/4 HP, max. flow 0.83 gpm) tojunction mixer 20 through line 46 (316 SS, 1/4 inch OD) and check valve88 (Whitey, 316 SS, 1/4 inch diameter). A single loop controller 90(Moore, Model 352E) is combined with pH transmitter 48 (Great LakesInstruments, Model 672P3FICON) and pH Probe 48A (Great LakesInstruments, Type 6028PO) to regulate the flow of acid to junction mixer20 via automatic flow control valve 50 (Research Controls, K Trim, 1/4inch OD, 316 SS) in response to the pH of the silicate/acid mixturemeasured at the exit of the junction mixer. An automatic three-way valve52 (Whitey, 316 SS, 1/2 inch diameter) is also employed within thecontrol system to allow for the possibility of having to divertoff-spec. silicate/acid mixture to the sewer. Dilution water from waterreservoir 10 is provided via line 54 (316 SS, 1/2 inch OD) to dilute theacid supply upstream of junction mixer 20 to a predeterminedconcentration in the range of from 1 to 20 wt. %. A static mixer 56(Cole-Palmer, 316 SS, 1/2 inch diameter, 15 turns) is provideddownstream of the point where dilution water is introduced into the acidsupply line to insure complete mixing and dilution of the acid. Arotameter 58 (Brooks, Brass Ball, 1.09 gpm. maximum), control valve 60(Whitey, 316 SS, 1/2 inch needle) and check valve 62 (Whitey, 316 SS,1/2 inch diameter) are used to control flow rate and flow direction ofthe dilution water.

The silicate/acid mixture which exits junction mixer 20 has preferably aSiO₂ concentration in the range of from 1.5 to 3.5 wt. % and a pH in therange of from 7 to 10. Most preferably the silica concentration ismaintained at 2 wt. % and the pH at 9. The mixture is passed through anelongated transfer line 64 (11/2 inch schedule 40 PVC pipe, 75 feet inlength) in route to finished product receiving tank 18. The length ofthe transfer line is selected to insure that the transfer will take atleast 10 seconds, but preferably from about 30 seconds to 90 seconds,during which time "aging" or partial gelation of the mixture takesplace. Transfer time can be as long as 15-16 minutes at very low flowrates and still produce satisfactory results. Dilution water fromreservoir 10 is added via line 66 (316 SS, 1/2 inch OD) to the mixturejust prior to its entry into finished product receiving tank 18 or atany other convenient location so long as the silicate/acid mixture isdiluted to an SiO₂ concentration of less than 1.0 wt. % which stabilizesthe gelation process. Dilution water is supplied with centrifugal pump68 (Eastern, 316 SS, 1 HP, 54 gpm maximum), and flow control isaccomplished at a predetermined rate with control valve 70 (Whitey, 316SS, 1/2 inch needle) and rotameter 72 (Brooks, SS Ball, 12.46 gpmmaximum). The finished product receiving tank 18 is provided with alevel control system 74 (Sensail, Model 502) which operates inconjunction with an automatic three-way valve 76 (Whitey, 316 SS, 1/2inch diameter) to divert flow of the silicate/acid mixture to the sewerif the level of finished product becomes too high.

After a period of continuous operation, which depends on the amount ofactive silica produced, it may be desirable to cease generation of theactive silica and flush the mixing junction 20 and that portion of thesystem which is downstream, i.e., piping, valves, transfer lines, etc.,which have been in contact with the silicate/acid mixture, with waterand warm NaOH. Flushing the system removes any undesirable silicadeposits which may have accumulated in parts of the apparatus where therequired turbulent flow conditions could not have been maintained due todesign restrictions, as for example in the region of pH measurement. Theflushing procedure helps maintain the system free of silica depositionand is begun by first shutting off dilution pump 68, acid pump 44 andsilicate pump 24. Dilution water from pump 36 is then circulated throughthe downstrean portion of the system for about 5 minutes, after whichpump 36 is shut off, and the dilution water reservoir is isolated byclosing valves 40, 60 and 70. Three-way automatic valves 52 and 76, andmanual valves 78, 80 and 82 (all Whitey, 316 SS, 1/2 inch OD) are thenactivated along with centrifugal circulating pump 84 (Eastern, 316 SS,1.5 HP, 15 gpm maximum) to allow NaOH, maintained at a concentration of20 wt. % and a temperature in the range of from 40° to 60° C., tocirculate through the downstream portion of the system for generally notlonger than about 20-30 minutes. The NaOH circulating pump 84 and theflush tank 16 are then isolated from the system by again activatingthree-way valves 80 and 82, and dilution water is again flushed throughthe downstream system and released to the sewer. Having completed thecleaning/flushing procedure, the production of active silica can beresumed.

Referring now to FIG. 2, there is shown a schematic diagram of a dualline production system for active silica, whereby one line can beoperational at all times while the other line is being flushed or beingmaintained in a stand-by condition. The component parts are numbered inaccordance with FIG. 1. A commercial system according to either of FIGS.1 or 2, will generally be constructed of stainless steel or polyvinylchloride tubing of generally one inch diameter or less, depending on therequirement for active silica. When stainless steel tubing is used,connections of the various instruments, fittings, valves, and sectionscan be conveniently made with "Swagelok" compression joints.

FIG. 3 is a schematic diagram showing a modification of the basicapparatus of FIG. 1 suitable for the production of polyaluminosilicatemicrogels. From the reservoir 100, a concentrated solution of analuminum salt, preferably aluminum sulfate, can be pumped through tubing(1/4 inch diameter 316 stainless steel) by means of a diaphragm meteringpump 102 (Pulsatron® Model LPR 2-MAPTC1, glass filled polypropylene,Teflon® diaphragm, max. flow 12.5 ml/min). The metering pump 102 can belinked electronically to the controller 90 and can move in parallel withsilicate usage. After passing through check valve 104 (Whitey, 316 SS,1/4 inch diameter), the aluminum salt solution can be introduced intothe diluted acid line at the point 106 by means of a 316 SS "T"junction. Thorough mixing of the aluminum salt with the diluted acid canbe completed by the in-line mixer 56 before reaction with the silicate,to produce polyaluminosilicate microgels, occurs at "T" junction 20. Apreferred aluminum salt solution for use in the method is a commercialsolution of aluminum sulfate such as liquid alum solution Al₂(SO₄)₃,14H₂ O containing 8.3 wt. % Al₂ O₃ supplied by the AmericanCyanmnid Company.

Periodically, it is necessary to flush the polyaluminosilicate apparatusfree from silica deposits by means of warm caustic soda solution asdescribed above.

It should be understood that a dual line apparatus for the continuousproduction of polyaluminosilicate microgels can be constructed by theappropriate modifications of the dual line apparatus of FIG. 2.

EXAMPLE 1

Demonstrating the effect of turbulence in reducing silica deposition.

A laboratory generator for producing polysilicate microgels wasconstructed according to the principles described in FIG. 1. Thesilicate and sulfuric acid feeds, before dilution and mixing, contained15 wt. % silica and 20 wt. % acid respectively. The critical junctionmixer was constructed from a 1/4 inch, 316 stainless steel "Swagelok"T-compression fitting fitted with 6 inch arms of 1/4 inch OD 316 SStubing. The internal diameter of the fitting was 0.409 cm. For the testsin which a gas was introduced into the mixing junction a similar"Swagelok" X-compression coupling was used with the fourth and of the Xas the gas inlet. An in-line filter comprised of 1 inch diameter 60 meshstainless steel screen was placed about 12 inches from the acid/silicatejunction to trap particulate silica. The screen was weighed at thebeginning of each test and again at the end of each test, after washingand drying, so as to give a measure of silica deposition. All tests wererun so as to maintain conditions of 2 wt. % silica and pH 9 at the pointof silicate acidification and each test was run for sufficient time toproduce a total amount of 1,590 gms. of polysilicate microgel. Theresults of the tests are given in Table 1 below. Liquid flow representsthe total liquid flow, that is, the flow of the combined silicate/acidmixture in the exit tube. In the tests where a gas was introduced toenhance liquid flow and turbulence, the Reynolds number was calculatedon the basis of the increased flow rate of the liquid portion alone,assuming that liquid density and viscosity did not change appreciably.This method of calculation was adopted because there is no ready formulafor calculating the Reynolds number of liquid/gas mixtures.

                  TABLE 1                                                         ______________________________________                                        Silica Deposition As A Function Of Reynolds Number                                            Run    Liquid Gas     Silica                                  Test Reynolds   Time   Flow   Flow    deposited,                              No.  No.        mins.  ml/m   ml/m    gms.                                    ______________________________________                                        1    1,036      330    250    none    0.339                                   2    2,072      165    499    none    0.135                                   3    4,144      83     999    none    0.009                                   4    6,217      55     1,498  none    0.007                                   5    10,362     33     2,497  none    0.002                                   6    12,433     27     2,996  none    0.008                                   7    12,260     120    694    Air, 2,260                                                                            0.008                                   8    9,064      120    694    Air, 1,490                                                                            0.005                                   9    5,375      120    694    Air, 601                                                                              0.004                                   10   5,375      120    694    N2, 601 0.014                                   ______________________________________                                    

A comparison of the results of Tests 1 & 2 with the results of Tests3-10 clearly demonstrate the beneficial effect of turbulent liquid flow(Reynolds number above 4,000) in reducing the amount of silicadeposition observed. Under turbulent flow conditions of the presentinvention, the average silica deposition of 0.007 g represented only0.0004% of the total amount of silica processed. When the Reynoldsnumber was below the minimum of 4,000 required by the instant invention,undesirable silica deposition was at least approximately 15-foldincreased. Once the minimum Reynolds number required by the process ofthis invention was reached, increasing the Reynolds number above 4,000,for example from 4,144 to 6,217 to 10,362, etc. did not appreciablyreduce silica deposition further.

EXAMPLE 2

Apparatus

A commercial sized apparatus for preparing active silica microgels wasassembled according to the schematic design shown in FIG. 1 andinstalled in a commercial paper mill. The apparatus, except for the rawmaterial supply reservoirs, was rigidly mounted on steel framework ontwo skids each measuring approximately six feet by eight feet. On skid 1was mounted inlets for connection to commercial supplies of sodiumsilicate and sulfuric acid and an inlet for city water which was usedfor dilution purposes. Also on skid 1 was mounted the dilution and flowcontrol means, the silicate/acid mixing junction, pH measurement and pHcontroller, sodium hydroxide flush reservoir, required pumps and valvesand the electrical controls. On skid 2 was mounted the aging loop,finished product reservoir, level controller and required pumps andvalves. Overall height of each skid was about seven feet. Themanufacturers supply containers were used as reservoirs for the silicateand sulfuric acid and these were connected directly to the appropriateinlets on skid 1.

The apparatus was operated continuously for six (6) days during which0.5 wt. % active silica was produced at a rate which varied between 3and 4.8 gallons per minute. At a production rate of 3 gpm, a Reynoldsnumber of 4250 was calculated for the mixing zone employed. No silicadeposition was observed within the junction mixer 20, although somesilica deposition was observed in the proxi immediately downstream fromthe junction mixer exit after 12 hours of continuous operation. Toalleviate this situation, a water/NaOH/water flush sequence wasconducted, which took less than 30 minutes, and the system was thenreturned to normal production. Over the entire six day period, theapparatus operated without fault and produced active silica of excellentquality which was utilized by the mill for the production of a range ofpapers with different basis weights.

EXAMPLE 3

Preparation of Polyaluminosilicate Microgel

A commercial-sized apparatus for preparing polyaluminosilicate microgelsolution was assembled according to the principles shown in FIG. 3. Theapparatus, except for the raw material supply reservoirs, was rigidlymounted on steel framework on two skids each measuring approximatelyeight feet by eight feet. On skid 1 were mounted inlets for connectionto supplies of sodium silicate, sulfuric acid, sodium hydroxide andpapermaker's alum and an inlet for city water which was used fordilution purposes. Also mounted on skid 1 were the required pumps foreach chemical and a reservoir for containing the finishedpolyaluminosilicate microgel solution. On skid 2 were mounted flowcontrol valves for sodium silicate, acid, and the dilution water, thesilicate/acid mixing junction, pH measurement means and pH controller,an aging loop, and a sodium hydroxide flush reservoir. Flow of thepapermaker's alum was controlled by a diaphragm pump at rateproportional to the silicate flow. The papermaker's alum was introducedinto the diluted acid stream prior to the silicate/acid mixing junction.The resulting polyaluminosilicate microgel solution had an Al₂ O₃ /SiO₂molar ratio of approximately 1/1250.

The apparatus was used to produce 6000 gallons of 0.5 wt %polyaluminosilicate microgel solution at a rate of 20 gallons perminute. A Reynolds number of 22,700 was calculated for the mixing zone.Only minor silica deposition was noted on the pH electrode after 5 hoursof operation. To remove the silica deposits, a NaOH flush was conducted,which took less than 30 minutes, and the system was then returned tonormal production. The polyaluminosilicate microgel solution wasutilized by a paper mill for the production of liquid packaging boardwith excellent results.

We claim:
 1. A method for continuously preparing a polyaluminosilicatemicrogel resulting in reduced silica deposition in which the microgelcomprises a solution of from 1 to 2 nm diameter silica particles havinga surface area of at least about 1000 m² /g which are linked togetherinto individual chains to form three-dimensional network structures saidmethod comprising:(a) simultaneously introducing a first streamcomprising a water soluble silicate solution and a second streamcomprising an acid having a pKa of less than 6 and a solution of analuminum salt into a mixing zone where the streams converge at an angleof not less than 30 degrees and at a rate sufficient to produce aReynolds number in the mixing zone of at least about 4000 and aresulting silicate/acid/salt mixture having a silica concentration inthe range of from 1 to 6 wt. % and a pH in the range of from 2 to 10.5;(b) aging the silicate/acid/salt mixture for a period of time sufficientto achieve a partial gelation, but not longer than 15 minutes; and (c)diluting the aged mixture to a silica concentration of not greater than2.0 wt. %.
 2. A method for continuously preparing a polyaluminosilicatemicrogel resulting in reduced silica deposition in which the microgelcomprises a solution of primary silica particles of from 1 to 2 nmdiameter silica particles having a surface area of at least about 1000m² /g which are linked together into individual chains to formthree-dimensional network structures said method comprising:(a)simultaneously introducing a first stream comprising a water solublesilicate solution and a second stream comprising an acid having a pKa ofless than 6 and a solution of an aluminum salt into an annular mixingdevice where the streams converge by the discharge of one stream from aninternal pipe of the mixing device into the second stream flowingthrough an external pipe at a rate sufficient to produce a Reynoldsnumber in the mixing zone of the mixing device of at least about 4000and a resulting silicate/acid/salt mixture having a silica concentrationin the range of from 1 to 6 wt. % and a pH in the range of from 2 to10.5; (b) aging the silicate/acid/salt mixture for a period of timesufficient for the primary silica particles to link together and formsaid three-dimensional structures while remaining in solution, but notlonger than 15 minutes; and (c) diluting the aged mixture to a silicaconcentration of not greater than 2.0 wt. %.
 3. The method of claims 1or 2 wherein the silica concentration in the resultingsilicate/acid/salt mixture is from 1.5 to 3.5 wt. % and the pH is from 7to
 10. 4. The method of claim 1 or 2 wherein the pH is from 2 to
 7. 5.The method of claims 1 or 2 wherein said silica concentration is notgreater than 1.0 wt. %.